1. Field of the Invention
This invention relates to liquid films and, more particularly, to a method and apparatus for generating a thin, freestanding liquid film which, in one application serves as an electron stripper for an ion beam.
2. Prior Art
Thin oil films have potential applications as electron strippers in heavy ion accelerators where high charge states are desired. An electron stripper is a device which removes additional electrons from an ion to increase the charge of the ion.
Previously, two techniques for electron stripping have been available, depending on the ion velocity. In one technique, called gas stripping, an ion beam is passed through an electron stripping gas or vapor. For example, the Super HILAC Linear Accelerator at the Lawrence Berkeley Laboratory uses a flourocarbon oil as the stripping medium vapor for 113 keV/A ions injected by the ABEL high intensity injector. In another technique, a higher energy ion beam is passed through a thin carbon foil. For example, in the same Super HILAC Linear Accelerator, a 35 microgram/cm.sup.2 carbon foil is used for ions accelerated to 1.1999 MeV/A. The carbon film technique produces higher charge states than does the gas stripping technique but the carbon films have limited lifetimes. For an average current beam of one microampere of Ar.sup.40, the lifetime of a carbon foil is one hour. With the use of higher ion currents and higher mass ions, that is, with atomic weight A between 100 and 238, now available, the lifetimes of carbon foils are considerably less, possibly less than one minute. Longer lived carbon foils, which have been produced by better carbon disposition methods, are available but they are difficult to produce and expensive.
A third technique which had the potential to solve the short lifetime problem of the carbon foils was suggested by Cramer et al in an unpublished article submitted Sept. 23, 1980 to Nuclear Instruments and Methods entitled "Production of Optically Thin Free-Standing Oil Films from the Edge of a Rotating Disc". In the Cramer et al article, a sharp-edged rotating disc touches the surface of an oil reservoir and spins a thin film from the edge of the disc. Longterm, stable operation was not achieved because the oil level in the reservoir changed so that equilbrium was not achieved. Vibrations also degraded the stability of the film and reproducibility apparently was a problem. The area of the thin film was limited and could be a problem in linear accelerators of the Super HILAC type where the ion beam wanders perhaps as much as a centimeter from a nominal beam-line axis.